Magnetic control of film deposition



Nov. 1, 1966 E. KAY ETAL 3,282,815

MAGNETIC CONTROL OF FILM DEPOSITION Filed July 1. 1965 5 Sheets-Sheet 1iSHZND CATHODEiLAYERS (NEGATIVE GLOW n L L- 1 i ASTON DARK SPACE 1FARADAYDARKSPACE mom: oRRR SPACE CATHUOE DARK SPACE v a z LIGHTINTENSITY I i I --Fr- -4 l POTENTIAL VC l I R V I 1 1 I I "IF-WM MW" 1.i v I l I cAs TEMPERATURE H 6 INVENTORS ERIC KAY BY ARTHUR P POENISCH ATTORNE Y Nov. 1, 1966 E. KAY ETAL 3,282,815

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Nov. 1, 1966 E. KAY ETAL 3,282,815

MAGNETIC CONTROL OF FILM DEPOSITION Filed July 1, 1963 5 Sheets-Sheet l.

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Patented Nov. 1, 1966 York Flled July 1, 1963, Ser. No. 291,736 7Claims. (Cl. 204-192) The present invention relates to a device for thesputtering of thin films and, more particularly, to systems forimproving such evaporation through the use of balanced magnetic fieldsto control the transport mechanism. Thin film technology, despite itscomplexity and some inherent problems, offers workers in the art anattractive approach for the controlled deposition of thin films. Thecrystal morphology, thickness uniformity, and isotropic growth ofcrystals in certain thin film materials, can be controlled withunexcelled precision using sputtering systems, sometimes called impactevaporation" systems and hereinafter referred to as such. This is ofespecial importance for magnetic applications since thesecharacteristics contribute vitally to the magnetic properties of theresultant film.

One disadvantage of prior art impact evaporation systems has been arelatively slow rate of deposition. Another and related problem is thelow ion current density of the eroding ions. The present inventionprovides a solution to these problems and enables workers in the art toincrease ion densities at least one order of magnitude and to increasedeposition rate even more, without introducing any serious new problems.

It has been realized that one way to increase the deposition rate ofsputtered material is to increase the effective path of the electronswhich generate the cathodebombarding ions, since the statisticalprobability of an ion-producing collision increases with the increasingpath of the electron. One suggested solution is to apply aunidirectional transverse magnetic field so as to curve these paths andhence lengthen them. However, this solution is an impractical one sinceit has the disadvantage of crowding the electrons, and thus thebombarding ions which they generate, to one side of the cathode andthereby destroy the symmetry of the deposited film. Symmetry isabsolutely essential for magnetic and other types of thin films. Thepresent invention provides a solution to he problem of increasing theelectron path without the attendant disadvantage of destroying filmsymmetry.

A second method of increasing the rate of deposition is to lower thepressure within the sputtering vessel and hence increase themean-free-path of the impact eroded particles diffusing to thesubstrate. However, a difficulty associated with this solution is thatthe Crookes Dark Space (CDS) elongates with decreasing pressure andhence, at some point, a masking etfect" occurs which destroys filmuniformity. This occurs because homogeneous ionization is elfectivelyprevented at a region where the substrate intrudes into the ionizationzone, since the substrate distorts the field there and blanks outerosion in an area subtending the intrusion. A decreased erosion,usually at the center of the cathode, results in a void of sputteredfilm material on the "intruding" substrate area. This is called amasking or shadow" effect. To avoid this, one must necessarily move theanode farther away from the cathode and thus lose the advantage of theextended mean-free-path. The present invention provides a means ofincreasing the rate of deposition without need for decreasing thepressure of the vessel and the consequent extension of the Crookes DarkSpace. One way wherein it accomplishes this is to increase ionizationand deposition rates by many times. Where prior art rates of depositionwere on the order of l A./second, with the invention about A./second ispossible. Because of such increased symmetric ionization, the dischargecan also be operated at lower pressures than possible without themagnetic field of the invention and still be compatible with thin filmsymmetry deposition requirements.

Sputtering systems which have been suggested use a magneticfield-generating means to extend the effective paths of the electrons.But these have necessarily involved placing this means inside thesputtering vessel and thereby enlarging the vessel while aggravating thepump down problem. The present invention not only provides a means forgenerating a novel magnetic field configuration, but also teaches atechnique that dispenses with the need for locating the means inside thevessel and hence introduces no pumping or contamination problems.

The present invention not only provides novel magnetic means forsputtering devices, but also teaches how such means may be manipulatedand positioned relative to the glow discharge to establishcharacteristic glow-zones (cf. FIG. 7) and move them relative to theelectrodes for optimized deposition.

In addition, the magnetic means of the invention serve to increasesputtering efficiency when located according to the invention so as tooptimally position the conventional zone of ionization relative to thefield.

A further novel effect of using said magnetic means is to create asecond and hitherto unknown zone of ionization which is also acontrolling factor upon the sputtering rate.

The present invention solves the above problems and teaches newadvantages to those skilled in the art of impact evaporation byproviding a glow discharge device using a means for producing a magneticfield which includes a strong radially-symmetric, transverse componentwhich increases particle density and rate of deposition, while filmuniformity is maintained by locating this component at the maximumionizaton zone.

Accordingly, it is an object of the present invention to improve therate of impact evaporation by superposing radially-balanced transversefields upon the discharge.

Such a balanced (or quadrupole) magnetic field has particular utilityfor cathode processes in an Abnormal Truncated Glow Dischargeenvironment and offers a new and useful control for thin film growthfrom such plasma. Such quadrupole magnetic fields impressed upon thesputtering discharge can be a useful means of locating, selectively,these zones of ionization. These zones result from electron-atomcollisions in the plasma and such fields may enhance the ionizationprocesess. Such quadrupole magnetic fields can insure a symmetricion-current-density profile across the cathode surface. These magneticquadrupole fields can also permit gas density, cathode fall potentialand ion current density to be changed independently of each other while,in the prior art, one of them is determined by the other two. Theapplication of such a field to a planar electrode configuration canreadily produce an orderoE-magnitude increase in ion current density atthe cathode under constant voltage and gas density. This extendsconsiderably the rate of erosion processes at the cathode and theresultant deposition rate at the substrate.

Therefore, a further object of the invention is to provide a quadrupolefield to overcome the characteristic low sputtering and deposition ratesat low pressures of about 10- Torr in a manner compatible with uniformthin film growth over a limited surface area. Increased rates arepossible because this quadrupole field discourages deleteriousback-diffusion to the cathode by inducing a longer mean-free-path of thecathode-ejected particles. The inventive sputtering environment movesout of the diffusion and into the molecular-flow transport mechanism,thereby eliminating most back-diffusion and resultant cathodecontamination.

A further advantage of the low-pressure sputtering enhanced by theinvention is that contamination of the film can be minimized. This iscrucial, for example, in producing superconductive films where a fewparts in one million of oxygen renders a film useless.

Yet another advantage produced by the inventive quadrupole or balanced,magnetic field is that of shortening the Crookes Dark Space and,therefore, making it possible to bring the substrate closer to thecathode without shadow eflects producing film non-uniformities.

Therefore, it is one object of the present invention to increase sputterdeposition rates, especially in low pressure environments by providing abalanced magnetic field properly located.

Another object of the invention is to increase particle density withoutinjuring sputtered film uniformity by providing a quadrupole transversemagnetic field.

Yet another object is to extend the effective path of the electrons andhence generate more impact ions for impact evaporation, by superposing aquadrupole magnetic field upon the discharge.

Still another object of the invention is to improve sputtering rates byimpressing a quadrupole magnetic field upon sputtering particles.

Another object is to optimally locate glow discharge zones usingmagnetic field means so as to enhance sputtering.

The foregoing and other objects, features and advantages of theinvention will become apparent in the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings, wherein:

FIG. 1 shows the overall sputtering apparatus;

FIG. 2 shows the general quadrupole field including coils;

FIG. 3 is a plot of the position of quadrupole field axis along the glowdischarge axis vs. various erosioncurrent levels associated therewithfor difi'erent field strengths;

FIG. 4 plots voltage vs. current of vessel wall for varies". magneticfields;

FIG. 5 schematically shows the means for plotting the curve in FIG. 4;

FIG. 6 shows the characteristic glow discharge regions and dischargeparameters along the length of the discharge;

FIG. 7 is a map of the quadrupole magnetic fields of the invention withthe field positions kept constant and cathode position shifted relativeto this position;

FIG. 8 is a map similar to that of FIG. 7 with the shape of the fieldsmodified;

FIG. 9 is a map similar to that of FIG. 8 with the shape of the fieldsfurther modified;

FIG. 10 is a map similar to that of FIG. 8 with the shape of the fieldsmodified further;

FIG. 11 schematically indicates an idealized magnetic field of thelongitudinal type;

FIG. 12 schematically indicates an idealized magnetic field of thequadrupole type;

FIG. 13 schematically indicates an idealized magnetic field of aunidirectional transverse type; and

FIG. 14 is a plot of experimental data, showing the dependance ofincidence ion current at the cathode upon the relative location of thetransverse magnetic field component.

The setting wherein the invention is performed is best understood byreference to FIG. 1 wherein is shown the overall glow dischargeapparatus configuration. The glow discharge apparatus is confined in avessel 9 which is pressure-resistant so as to accommodate evacuation topressures in the range of 10" to 10 Torr. Vessel 9 may be made of metalso as to readily distribute an electric charge.

In this embodiment, vessel 9 comprises a large glass tube of theconfiguration as shown in FIG. 1, in which the walls are at least 2 fromthe perimeter of the anode to minimize wall-effects, as describedhereinafter. A bell jar could be used alternatively, but is not asappropriate since several ports, such as ports 98, 98' should beprovided in the vessel wall as far from the discharge as possible andyet allow coils 91, 91' to be close to the discharge.

The cathode 7 of this two-electrode glow discharge apparatus is planarand made of the material to be transported, i.e., deposit material.However, it may optionally be merely overlaid with a sheet of thedepository material, the basic configuration of the cathode being keptstandard. A substrate is affixed upon the face of anode 10. Anode 10 is,of course, adjustable in the cathode-anode axial direction so as toallow a change of axial location of the substrate mounted thereon. Injoining substrate 90 to anode 10, it is important that the connectionshould provide good thermal conduction. Anode 10 may be of anyconductive, heat resistant metal, such as aluminum. Both cathode andanode are water cooled so that their temperatures can be maintainedconstant and, if desired, as low as room temperature. Both electrodesare about 4V2 inches in cross-sectional area. Cooling units are providedin jacket form within the base 6 of the cathode 7 and also within theanode 10. Any suitable coolant such as water may be pumped in at thecathode inlet 4, emerging at outlet 5 to be cooled and recirculated.Likewise, inlet 11 and outlet 12 provide the coolant for the anode 10.The cathode-anode potential drop can be varied between 0-5000 ev. usinga SKV-SOO ma. low impedance filtered DC. power supply. Desiredglow-discharge effect occurs only in the pressure range of 10' to 10-mm. of Hg. But, in order to maintain a glow discharge in the lowerpressure regions, where the mean-free-path of electrons is long, theionization efficiency has to be increased. This may be done in severalways. One way is by superimposing a transverse external magnetic fieldon the discharge, thus increasing the efiective path of the collidingelectrons and hence the probable number of bombarding particlesgenerated by these collisions. The quadrupole transverse field producedby coils C C accomplishes this according to the invention as notedbelow, and additional-1y prevents asymmetrical deposition. This field isbetter understood by considering FIGS. 11, 12 and 13 where it isgenerated by coils D.

The longitudinal, quadrupole magnetic fields produced by the twoseparate water-cooled coils C C; are coaxial with the discharge andradially symmetric about it, influencing it considerably as explainedhereinafter. The direction (polarity) of the magnetic fields can bechanged by changing the direction of the current in the individual coilsas noted in Table II. When the two quadrupole coils are arranged inopposing polarity, as is the preferred arrangement (see FIG. 2, C Cm), atransverse field region, designated hereinafter as Line X (see FIG. 2),will occur where the axial components of the buclting fields cancel out.The locus of this region may be changed by adjusting the relative fieldstrengths (coil current) or moving the coils. As FIGS. 7-10 demonstrate,

the location of this region sharply influences deposition. The presentinvention involves teaching the provision and proper location of such atransverse field of the radially symmetric type, as particularlydescribed somewhat later.

A unidirectional transverse field, on the other hand, may be providedfor this sputtering apparatus by separating the coils and placing one oneach side of the apparatus with their axis perpendicular to that of theelectrodes, as schematically indicated in FIG. 13.

For establishing a sputtering-vacuum," a diffusion pump is used to pumpdown to glow discharge pressures through outlet 14 and thereaftermaintain constant pressure while clean flushing gas is being fed at port15. But, ion current density is very sensitive to small pressurefluctuations. Therefore, the fiow rate of inert gas through the systemmust be closely regulated by balancing the gas input from a reservoir(not shown) through port with the output through outlet 14 to the pump.This is done by balancing the gas input through a variable leak, forinstance. a double-needle valve against the pumping speed of thediffusion pump connected to outlet 14. This is superior to merelythrottling the diffusion pump as done in the prior art. This diffusionpump is of the oil diffusion type and nitrogen trapped, having acapacity of about 700 liters per second and able to achieve a minimumpressure of about 1X10- mm. Hg in this glow discharge chamber (i.e., inFIG. 1), The pressure is continuously monitored on the thermo-couplegauge which has been calibrated for argon gas using a McLeod gauge as areference.

The glow discharge zones are contalned according to the invention by theuse, both of the quadrupole magnetic field and of appropriately chargedand shaped shield ing means. Such a shielding means 8 is shown in FIG. Ias generally of a cup-configuration. When placed around planar cathode7, shield 8 prevents discharge except normally from cathode face 7,toward anode 10. The shielding must be placed within the Crookes DarkSpace (CDS) distance from the cathode assembly 6 or 7 to assure adischarge-free area between the cathode and the shielding. The anode 10,whose configuration is not critical, can be placed at varying distancesfrom the cathode, within the range, for instance, of 6 mm. to 2.5 cm.,beyond the shadow-zone" it should be as close as poss'ible foretficiency. The common glow discharge characteristics (e.g., current,pressure, voltage and geometry) are intenrelated by well-establishedsimilarity laws. Such laws are explained, for instance, in theEncyclopedia of Physics, edited by E. Flugge (Springer-Verlag, Berlin,Germany, 1956, vol. 22). These zones and parameters are schematicallyplotted in FIG. 6 and described below.

Analytical evaluations of the films indicated superior sputtering withthe inventive system and helped to monitor the control techniquesintroduced according to the invention.

Before describing the details of these sputtering control techniques, itis useful to consider the parameters whereby their effectiveness ismeasured. One such parameter and an important gauge of sputteringsuccess is film thickness profile. The comprehensive nature of thisproperty as an analytical method of delineating the uniformity ofseveral film growth parameters makes it one of the most useful criterionfor setting limits of control over these parameters. On a homogeneoussubstrate surface with no temperature gradients, areas of uniform filmthickness qualitatively imply uniform rates of arrival of incidentparticles. Uniform incidence leads to uniform particle size and shapedistribution in the resultant film, as well as crystallographicuniformity. Control over these particular parameters is especiallyimportant in the study of magnetic properties, as well as for all thinfilm production.

With commonly used planar glow discharge electrodes, having theparallel, planar electrode configuration (cf. FIG. 1), the thicknessprofile on a substrate will depend on a precise control of the transportmechanism of sputtered particles from the source (cathode) to thesubstrate.

The experimentally measurable quantities upon which the depositionthickness profile will depend are (1) the ion energy, current density,direction of incidence and the resultant erosion profile at the cathodesurface; (2) the location of the end of the CD8; (3) the magnitude ofthe cathode fall potential; (4) the pressure and subsequentmean-free-path of sputtered particles; and (5) the proximity of theenvelope walls with respect to the substrate. The parameters above areall related to one another and variation of any one of them will reflectitself in the thickness profile at the substrate in a logical fashion.

Related to deposition profile is the profile of the cathode erosionwhich generates the deposition. Further, the configuration of theerosion profile depends upon the ion current density profile. Aradially-nonuniform ion current profile at the cathode will result inradially nonuniform cathodeerosion and deposition profiles. The ioncurrent profile at the cathode will, in turn, depend on the electricfield distribution near the cathode. This field distribution dependsupon (1) the geometry of the cathode assembly and its position relativeto the apparatus envelope; and (2) the distance of the substrate andanode assembly to the cathode. The invention specifies these parametersbelow, as optimized by the quadrupole field superposition.

Further details on the general incidents of sputtering and details ofapparatus suitable for use with the invention herein may be found inco-pending US. application, Serial No. 290,794, filed June 26, 1963entitled Deposition of Thin Film by Impact Evaporation, by Eric Kay,assigned in common with this case. Also relevant is the article:Magnetic Field Effects on an Abnormal Truncated Glow Discharge and TheirRelation to Sputtered Thin Film Growth, in the Journal of AppliedPhysics, April 1963.

The rate of film deposition is a very important thin film preparationparameter. It depends on the rate of ejection of particle from thecathode (source) at a fixed pressure and incident ion energy. The rateof ejection (or sputtering) from the source, in turn, depends mainly onthe high energy incident ion current which gives rise to sputtering. Themagnitude of this ion current depends upon the degree of ionization inthe plasma. This ioniza tion process can be increased considerably in amanner compatible with thin film technology only with the superpositionof a symmetric quadrupole field of the invention. According to theinvention, the superposition of the transverse component of a quadrupolefield on certain narrow regions of the discharge plasma (optimumionization zones) leads to the enhancement of sputtering and thesubsequent deposition rate is optimized. We find that superposition ofthis type of field can move and attenuate the conventionally-known zoneof optimum ionization. However, it also creates a second, new such zone,one of which is completely absent when no magnetic field is presentwhich has never been reported before. The first narrow ionization zone(nearest the cathode) which is well known from earlier literature, movesconsiderably toward the cathode when this particular type of magneticfield is applied according to the invention. Since the effectiveionization zones are only a few millimeters wide, it is obvious that theposition of the transverse component of the magnetic field with respectto these zones is critical. The exact position of the two criticalionization zones is a function of many parameters such as voltage,pressure, magnitude of magnetic field in the plasma region, etc. Thespecific quantitative relation of these variables to zone position inany plasma is not yet known; however, the importance of properlylocating these zones and applying the inventive quadrupole magneticfield as taught herein is unquestionable.

An actual plot of a representative version of the symmetric quadrupolefield used is shown in FIG. 2 with the absolute field strengths alongthe indicated "equigaussian" lines are noted along the,horizontal axisof symmetry. This quadrupole field of configuration was produced byopposing field coils C and C and was plotted with a Hall probe, theabsolute gaussian field values being given there. It can be seen thatthe major transverse component of the field (Line X) is most pronouncedalong the axis of symmetry. Hacking the cathode 107 away from Line Xmoves the well defined plasma zones (in a given discharge) lying infront of the cathode past Line X and modifies the deposition mechanism(cf. explanation below for FIGS. 7-10). As the major transverse fieldcomponent is made to approach the region near the CDS-NG transition(shown in FIG. 6, for example), a maximum in ionization is predicted,since in this region are found a high density of electrons in an optimumenergy range for ionization. This major transverse field component, LineX, will cause such electrons to elongate their paths (spiraling) andthus increase the number of ionizing collisions and, in turn, acceleratesputtering. Such increases are indicated in FIG. 3 at points 8" whichrepresent ion-current maxima. Moreover, since the major transversecomponents are balanoed according to the invention, this increase isachieved without destroying film uniformity.

Each of the three curves in FIG. 3 represents the variation of ioncurrent (arriving at the cathode to erode it) as a function of cathodeposition (D) with respect to the location of the maximum transversemagnetic component (Line X in FIG. 2). All other glow dischargeconditions are kept constant, of course (e.g., geometry, voltage,pressure). This location is determined either by adjusting relative coilcurrent (cf. FIGS. 7-10) or by the position of the coils along thedischarge. As expected, the ion current increases considerably withincreasing magnetic field. (Compare curves I, II and III.) Thus, thestrength of the quadrupole magnetic field can determine the size andlocation of this optimum ionization zone according to the invention andthen the coils may be positioned so as to superpose the maximumtransverse field components (Line X) upon this zone and thereby increasethe sputtering rate.

Curves I, II and III also indicate that, besides manipulating theconventional optimum-ionization zone (zones of the B maxima), thesymmetric quadrupole field of the invention gives rise to a second novelzone of optimum ionization (points A). This second zone is completelynew and unexpected and appears to be capable of producing the greatestincrease in the sputtering rate. This is especially evident from theincident ion current-Line X position" curve in FIG. 14 (of. point A).This curve was derived from 2000 volt, 31 micron pressure sputteringconditions using an aluminum cathode. It is evident that when the majortransverse field component (Line X) was positioned at this new secondionization zone (point A), the enhancement in incident ion current wasconsiderably greater than at the conventional first zone (point B).Removal of the quadrupole field entirely leaves a much lower, constantion current level (point C).

Reverting to FIG. 3 further shows that this second, new zone is asharply-defined zone of ionization (zone A). It depends upon electroncollision phenomena occurring several millimeters beyond the first zone(zone B), i.e., deeper in the NG region where electrons of suitableenergy for ionization can be confined by the superposition of thequadrupole magnetic field. With increasing magnetic field, this zonebecomes narrower 5 mm.see Curve III). The major transverse magneticfield component (Line X) can be moved back and forth along the dischargeaxis by producing a quad rupole field which, unlike that of FIG. 2, isasymmetric along the discharge axis 'but yet maintains a radial symmetrytransverse to this axis. The fields in FIGS. 7-10 exhibit thisasymmetry.

It is important to locate these narrow (few mm.) ionization zonesexactly before sputtering so as to locate the inventive field means atone of them. It will be evident that a preliminary trail discharge" forplotting ion current against the position (or relative current) of thecoils, as demonstrated above, will accomplish this.

Points A on Curves I, II, and III in FIG. 3 particularly indicate thatthis critical zone of ionization moves closer to the cathode withincreasing magnetic field. This is compatible with the observation thatthe CDS contracts under increasing field strength as discussed above.The shortened CDS will result in a reduction in charge transfer whichresults in higher energy incident ions at the cathode. This will bereflected in an increased sputtering rate at the cathode and, to aslightly lesser extent, a concomitant increase in the volume ofdepository material arriving at the anode. Inspection of Table I, below,in fact shows that for a 19-fold increase of incident ion current due tothe superposition of quadrupole field, the cathode erosion rate hasincreased 34 times and the deposition rate increased almost as much.

TABLE I Discharge data with and without quadrupole magnetic field CaseAI (lass B s uadrupole Magnetic Field, Amps 0 20430 oltage,V 2, I) 2,000Pressure Torr 3 2x10 3.2 10 Anode Temperature, C I8 18 ElectrodeSpacing, cm. 6 6 Lin Current, ms 8 Cathode Erosion Rate, mgJs 1. ti b4Anode Deposition Rate, m nis... 0. 56 17 Transport Elficiency, percentwt... A, 35 32 Sputtering Ratio, atoms/ion 0. rs 1. 26

It is interesting to note that the sputtering ratio, i.e., particlesejected per incident ion, increases from 0.77 to 1.26 when the field isapplied (case B). This is further evidence that higher energy ionsarrive at the cathode when the magnetic field of the invention isapplied. The anode deposition rate is directly related to the erosionrate at the cathode and is not adversely affected by the application ofa symmetric quadrupole magnetic field, because this field makes erosionsymmetrical. The few particles that become charged would not be afiectedby the small magnetic field used here at any rate.

In summary, the evidence above demonstrates the new and improvedsputtering elfects achieved according to the invention. They show thatsuperposition of a transverse magnetic field upon a sputteringdischarge, so that the field is radially symmetric about the dischargeaxis, generates a new optimum ionization zone. Further, positionin-g themajor transverse component of the field so as to overlie either zone ofoptimum ionization enhances cathode erosion, doing so in a radiallysymmetric manner and thus assuring deposition which is faster and moreuniform than the art has hitherto known. While the quadrupole magneticfield has been described in the chosen embodiment, it will be apparentto those skilled in the art that any balanced or radially symmetrictransverse magnetic field superimposed on the discharge according to theinvention should derive these benefits. For example, one mightalternatively revolve. at high frequency, a unidirectional transversefield, either mechainically or electrically around the common axis ofthe planar electrodes.

Positioning of the major transverse component of the magnetic fieldrelative to the selected ionization zones along the discharge may, asmentioned earlier, be effected either by physically moving the fieldcoils relative to the discharge elements or by changing the relativefield strength of the coils (by adjusting coil current). The latter isdone for the cases in Table II, below, relating ion current to effectivecathode position for the field maps of FIGS. 7-10. These magnetic fieldmaps in FIGS. 7-10, along with the conditions indicated in Table II,demonstrate how the major transverse magnetic field component can bemoved up or down along the discharge axis by producing a quadrupolefield which is asymmetric along the discharge (electrical field) axis,but yet maintains a radial symmetry along the horizontal axis. Thefields are indicated as actually measured by the iso-gaussian linesplotted in FIGS. 7-10, wherein the transverse components are evidentthough somewhat schematically. Comparing the relative coil currents (inTable III) with the coil interface location for these maps, demonstrateshow Line X may be moved in this manner without changing coil positions.

TABLE II Variation of cathode current (1 with magnetic field (see FIGS.7, 8, 9,

1 Longitudinal Field.

FIGS. 7-10 also indicate the existence of the abovernentioned zones ofoptimum ionization and demonstrate, by some typical test readings, thecriticality dependence of ion current upon proximity of these zones tothe major transverse component (Line X) according to the invention.

Some appreciation of the effect of the quadrupole field upon ion currentmay be had by comparing the ion current of Case VII (no magnetic field)with that of Cases I, II, and III (quadrupole field superposed infavorable locations). Cases III and IV are ignored since the field wasso located as to be relatively ineflective. This comparison not onlypoints up the advantage of the field per se, but also the criticality oflocating Line X properly according to the invention.

Case I (FIG. 7) demonstrates that as the cathode is moved toward Line X(evidently in the 3060 mm. region, though probably curved somewhat), ioncurrent increases markedly. Case II (FIG. 8) confirms this. It might bepointed out that, here, the cathode is effectively" relocated (atpositions a, b, c, a, b, c') by moving the coils (line P is theirinterface) since this is more con venient. This, of course, effectivelymoves the cathode relative to the field and hence changes the separationdistance between the ionization zones and Line X.

Cases III and IV (FIGS. 9 and 10) clearly indicate the importance oflocating the cathode in the right position with respect to the majortransverse field component. In neither case is this component closeenough to either of the major ionization zones to be effective. Theresult is that very little increase in ion current is observed. One mayconclude that it is of little advantage to impose a magnetic field on asputter discharge unless it is located propenly as taughtby theinvention.

The cathode ion currents (I listed in Table II also indicate theexistence of both ionization zones, and their influence upon cathodecurrent when the transverse field is moved near them. In FIG. 7, onlythe ionization region nearest the cathode is affected as the cathodeitself is moved past the major transverse field component. Comparing MapI with Case I, at D=6 mm. (Line c), neither of the ionization zones see"the transverse field component (Line X) so that ion current is not toodifferent from the "no-magnetic-field case (Case V). However, at D=32mm. (Line a), the ionization zone nearest the cathode does begin to seethe transverse field with the ion current more than doubling. Comparingthe conditions of FIG. 8, it will be observed that three cathodepositions (a', b, c) are identical to FIG. 7, only Line X being moved,effectively, with similar etiects upon ion current. Using similarreasoning, it can be seen that only the second ionization zone sees" thetransverse component, since as this zone is pulled farther away from thetransverse component the current goes down uniformly with no secondpeak.

Case V, on the other hand, demonstrates a much greater increase in ioncurrent as a result of locating the new, second ionization zone nearLine X. FIG. 2 may be compared for these field conditions, as they aresimilar to those in FIGS. 2 and 3 (see especially Curve II, peak A).This great increase demonstrates the advantage of finding this secondzone and using it according to the invention.

Thus, FIGS. 7-10, in conjunction with Table II, corroborate theinventive teaching that a radially symmetric transverse magnetic fieldmarkedly increases sputtering rates when it is located close enough toone of the above mentioned ionization zones. Further, the location isquite critical, ion current depending somewhat exponentially uponseparation from Line X. Thirdly, one of these zones is created by thefield itself and yields a greater sputtering improvement when locatedcontiguous to Line X.

Associated with the above described magnetic field means of theinvention is the novel technique of charging the vessel to a prescribed"wall potential so that the net current flow to the container walls canbe reduced to zero. Zero wall-current leads to higher ionizationethciencies, since no charged particles are trapped there and wasted. Italso results in a significant reduction in contamination problems sincethe wall-current particles" could unleash contaminants occluded to thewall. This is quite critical for some thin films.

The importance of charged particle current flowing to the wall, withrespect to thin film growth, will become evident. One way of preventingthis is by creating a magnetic wall in the discharge region.Consideration of FIG. 4 demonstrates the significance of this "magneticwall" on the net current flowing to the container wall. The vessel wall,in this case, was a metal liner of 6 inches diameter and, as indicatedin FIG. 5, was held at various voltages with respect to the potential.Purely longitudinal fields (shown in FIG. 11) of several hundredoersteds can provide such a magnetic wall which, in fact, reduces thenet current to the container wall to zero even when the wall is at apositive potential of several volts (Line S, FIG. 4). In an analogoussituation with no magnetic field. almost all the current would flow tothe walls instead of the anode as seen in FIG. 4 (Line R). Thisindicates that the longitudinal magnetic field is keeping the electronsaway from the wall as discussed earlier, in spite of the positive wallpotential, the latter being necessary to reject the positive charges,which are essentially unaffected by the magnetic field. When the wall iskept negative with respect to the anode, the wall current should be adirect measure of ion current being attracted to the wall.

When using the quadrupole field of the invention, however, (curves M, N,O) the situation is somewhat different since charged particles willagain be directed against the wall but the longitudinal magnetic fieldcannot be used to direct them away as above since this field woulddistort the quadrupole field, destroying it usefulness. Hence, thesecond means for preventing wall-current effects must be invokedaccording to the invention; namely, charging the vessel wall. It hasbeen found that the magnitude of wall-potential necessary is a functionof the strength (cf. FIG. 4 curves M and N) and location (com-pare curve0 with curve M-different cathode locations, hence different dischargezone locations under identical fields) of the major transverse fieldcomponent. FIG. 4 demonstrates the inventive teaching that by correctchoice of the wall potential, the net current flow to the containerwalls can be reduced to zero. By comparing curves M and N (FIG. 4), itcan be seen that, as the magnetic field is in creased, electrons arebeing bent more etfectively to the wall so that a higher negativepotential is required to reject them. Comparing O with M indicates thatthe position of the well-defined discharge zones (varied by changingcathode position D), relative to the field shape and strength, will alsodetermine to what degree elec trons are bent to the wall which, in turn,will dictate how negative a potential is needed on the wall to rejectthem.

From the measurements shown in FIG. 4, it can be inferred that thesputtered metal deposit on the glass wall (used in most of the otherexperiments reported here) will float at a slightly negative potentialexcept in the case of a longitudinal magnetic field where most electronsare being kept from the wall.

Thus, it is prescribed according to the invention that sputteringconditions may be improved, not only by a transverse, radially symmetricfield upon the discharge in a particular manner, but also by chargingthe vessel walls so as to draw zero net current thereto. With thequadrupole fields of the invention, this potential has been seen to be afew volts negative, varying according to the field strength.

While the particular embodiments described above represent usefulapplications for the inventive magnetic field for sputtering thin,uniform films, such uses do not exhaust the possible applications of theinvention, but will only serve to suggest others to those skilled in theart. Workers in the art will recognize, for instance, that the magneticfield of the invention is also advantageous for use with vacion(sputtering) pumps, since erosion and deposition rates are greatlyincreased, improving efiiciency at both the upper and lower pressurelevels.

In the broad sense, the inventive combination provides a means forimproving ionization efficiency and, consequently, sputtering efiiciencyalso, while maintaining erosion and deposition uniformity by extendingthe effective path of a sputterinitiating'electron without lengtheningits axial excursion and doing this in one of two advantageous dischargezones. Such a capability is advantageous in a multiplicity of plasmaenvironments, one of which is the sputtering environment, as describedabove.

One alternative application for the invention is in plasma nesearchwherein the better understanding of and more accurate control over glowdischarge mechanisms ofiered by the invention makes the dischargeenvironment a better diagnostic tool for plasma study. ilasma study iscurrently being hotly pursued as an aid in understanding thecharacteristics of charged particles for such applications as glowdischarge lamps, gaseous lasers, high energy physics, etc. A closelyrelated study area is research in the mechanics of secondary electronemission and of ion bombardment since ion bombardment causes secondaryemission in the usual sputtering context and since the invention willhelp in controlling these. A study of a sputtered film is, indeed, theonly practical way to distinguish sputter-ion current from sputteredsecondary emission at the cathode.

The invention also provides a new way of locating ionization zones in aglow discharge, as well as a method for creating a new zone ofsputter-ionization.

The invention offers workers in the film growth and crystalline studyarts a broader range of deposited-film characteristics by providing abroader range of erosion and deposition rates.

The invention has utility beyond the film deposition arts, being usefulfor sputter-erosion as well. By increasing the erosion rate radically,it may be advantageously used for ion-etching (i.e., erosion of asurface by ion-bombardment through sputtering). Ion-etching is usefulfor very finely controlled leveling or the roughening (e.g., to improveadherence) of surfaces according to the selection of the energy and massof incident ions.

For certain materials, such as refractories or multicomponent alloyswhich are ditficult or impossible to deposit by other methods,sputtering is the most practical, if not the only method of filmproduction. Vacuum evaporation, for instance, is not feasible with suchmaterials. Hence, for these types of films, the invention providesradically increased deposition efficiency and wider versatility indeposition techniques.

Other applications for the invention will likewise suggest themselves tothose skilled in the art.

While there have been described above and shown in the drawings varioussystems and methods for uniformly dispersing sputter electrons andimproving their cfiiciency in an advantageous discharge zone and therebyimproving the efficiency of the sputtering system and the uniformity ofdeposited films according to the invention, it is apparent that thevarious elements and steps may be modified or completely supplanted bythe use or substitution of other known elements or arrangements ofcomponents within the skill of those versed in the art. Accordingly, theinvention should be considered to include all modifications, variationsand alternative forms falling within the scope of the appended claims.

We claim: 1. In a method of coating an article by sputtering includingarranging said article within a closed vessel having a cathode and ananode and imposing within said vessel a suitable pressure of ionizablegas and a suitable anodecathode voltage potential difference to therebyobtain abnormal glow discharge conditions therein, said dischargecomprising electron-collision ionization of said ionizable gas and, inturn, ion-bombardment and impact-erosion of a surface of said cathode,as well as secondary electron emission at said cathode. said dischargethereby occurring about an axis centrally located and perpendicular tosaid surface of said cathode, the improvement comprising the steps of:

superposing a magnetic field having a major transverse componentsubstantially perpendicular to said discharge axis and radiallysymmetric thereabout;

locating said major transverse component of said field at a point alongsaid axis between said cathode and said anode within 60 mm. from saidsurface of said cathode to thereby increase the mean free path of saidelectrons; and

positioning said article in the negative glow region of said dischargeand within the bounds formed by said surface of said cathode, saidbounds being viewed through forming a cylindrical surface extendingparallel to said axis, to thereby provide uniform deposition thereon.

2. The method of claim 1 wherein said magnetic field comprises aquadrupole field.

3. The method of claim 2 wherein said locating step additionallycomprises:

locating said major transverse component of said field at the anode sideof the Crookes Dark Space generated by said discharge.

4. The method of claim 3 including, additionally, the step of chargingthe inner surface of said vessel so as to repel electrons and chargedparticles such that the net current flow to said surface is reduced tozero, thereby keeping them from dissipating against said surface, andthus improving the ionization rate and deposition rate and lowering theamount of contamination in the deposition due to sputtering of saidsurface.

5. The method of claim 2 wherein said locating step comprises:

sweeping said major transverse component along said axis between saidcathode and said anode;

detecting incident ion current at said cathode as a function of theaxial position of said major transverse component while keepingdischarge conditions otherwise constant; and situating said majortransverse component at approximately one of the detected positionsdefining a peak of incident ion current. 6. The method of claim 5wherein said situating step comprises:

situating said major transverse component at approximately the positionof maximum incident ion current. 7. The method of claim 2 wherein saidlocating step comprises:

locating said major transverse component of said field at approximatelythe position along said axis where application of said field results inmaximum incident ion current at said cathode.

5 References Cited by the Examiner UNITED STATES PATENTS 2,219,61112/1940 Berghaus et a1 204192 2,305,758 10/1942 Berghaus et a1 204-29810 JOHN H. MACK, Primary Examiner.

R. MIHALEK, Assistant Examiner.

1. IN A METHOD OF COATING AN ARTICLE BY SPUTTERING INCLUDING ARRANGINGSAID ARTICLE WITHIN A CLOSED VESSEL HAVING A CATHODE AND AN ANODE ANDIMPOSING WITHIN SAID VESSEL A SUITABLE PRESSURE OF IONIZABLE GAS AND ASUITABLE ANODECATHODE VOLTAGE POTENTIAL DIFFERENCE TO THEREBY OBTAINABNORMAL GLOW DISCHARGE CONDITIONS THEREIIN, SAID DISCHARGE COMPRISINGELECTRON-COLLISION IONIZATION OF SAID IONIZABLE GAS AND, IN TURN,ION-BOMBARDMENT AND IMPACT-EROSION OF A SURFACE OF SAID CATHODE, AS WELLAS SECONDARY ELECTRON EMISSION AT SAID CATHODE, SAID DISCHARGE THEREBYOCCURING ABOUT AN AXIS CENTURALLY LOCATED AND PERPENDICULAR TO SAIDSURFACE OF SAID CATHODE, THE IMPROVEMENT COMPRISING THE STEPS OF:SUPERPOSING A MAGNETIC FIELD HAVING A MAJOR TRANSVERSE COMPONENTSUBSTANTIALLY PERPENDICULAR TO SAID DISCHARGE AXIS AND RADIALLYSYMMETRIC THEREABOUT; LOCATING SAID MAJOR TRANSVERSE COMPONENT OF SAIDFIELD AT A POINT ALONG SAID AXIS BETWEEN SAID CATHODE AND SAID ANODEWITHIN 60 MM. FROM SAID SURFACE OF SAID CATHODE TO THEREBY INCREASE THEMEANS FREE PATH OF SAID ELECTRONS; AND POSITIONING SAID ARTICLE IN THENEGATIVE GLOW REGION OF SAID DISCHARGE AND WITHIN THE BOUNDS FORMED BYSAID SURFACE OF SAID CATHODE, SAID BOUNDS BEING VIEWED THROUGH FORMING ACYLINDRICAL SURFACE EXTENDING PARALLEL TO SAID AXIS, TO THEREBY PROVIDEUNIFORM DEPOSITION THEREON.